Plasmonic materials are metals or metal-like materials in which valence electrons are able to move freely. Common plasmonic materials include gold (Au) and silver (Ag). When incident light (e.g., from a light beam) interacts with a plasmonic material, stimulated valence electrons on the surface of the material (sometimes referred to as plasmons or surface plasmons) collectively ripple to create electromagnetic waves on the boundary between the surface of the material and the surrounding medium (e.g., air or water). Resonance occurs when the frequency of photons in the incident light matches the natural frequency of the oscillating surface electrons in the material. The response amplitude of an object or device composed of plasmonic material reaches a maximum at the resonance frequency (or wavelength).
Plasmonic nanostructures are objects or devices of a nanoscale size (e.g., an intermediate size between microscopic- and molecular-sized objects) that are composed of plasmonic material. Plasmonic nanostructures exhibit high internal energy density when irradiated with light. This high internal energy density arises from the coupling of their resonant free electron oscillations to the incident light. Plasmonic nanostructures can also be spectrally tailored. For example, the plasmonic resonance of nanostructures can be tuned from the ultraviolet through the visible and infrared spectrum based on design parameters such as the size, shape and architecture of the plasmonic nanostructures, which influence the excitation and propagation of plasmons. Accordingly, plasmonic nanostructures have been the subject of scientific research relating to various fields and topics, including subwavelength optical confinement, nanoscale photonic circuits, concentration schemes for photovoltaics, field enhancement for Raman spectroscopy, biological labeling techniques, and metamaterials.
Attempts to extract the high energy density of the excited electrons of an irradiated plasmonic nanostructure, for example, to drive an electric current through a circuit load, have exhibited low optical-to-electrical power conversion efficiency. An early approach is detailed in Knight, M. W., Sobhani, H., Nordlander, P., & Halas, N. J., Photodetection with Active Optical Antennas. Science, 332(6030), 702-704. doi:10.1126/science.1203056 (2011), the entire content of which is incorporated herein by reference. In some published examples, the optical-to-electrical power conversion efficiency was less than 1%. This poor power conversion efficiency is due in part to the optical behavior of metals and other plasmonic materials, which are characterized by large free carrier density. One of the primary reasons for the low optical-to-electrical power conversion efficiency is the very short excited state lifetime of electrons in metals and other conductors, which is usually less than 10 fs.
Previous power conversion schemes have generally featured transport of excited electrons over some type of semiconductor-metal interface to rectify the excited electrons in the current. However, such conversion schemes cannot be readily optimized for an appropriate interface barrier height or transit time, because it is challenging to move optically excited electrons through an electrical circuit with sufficient speed and optimal efficiency. For instance, fast electronic relaxation via electron-phonon coupling poses challenges to advancing the efficiency of hot-carrier collection. In addition, challenges arise because the characteristic energy of an optically excited electron in a plasmonic resonance is not known.